Which Scenario Illustrates a Selfish Herd

Which scenario illustrates a selfish herd?

A) In most wolf packs, one dominant male breeds with one dominant female.
B) Lions are social animals who cooperate in catching prey.
C) When threatened by wolves, musk oxen stand back to back, presenting an imposing display of sharp horns.
D) Chimpanzees learn by imitation in a group.
E) A pair of breeding herring gulls will cannibalize the eggs and even the chicks of their neighbors.